ORIGIN OF THE TRABBOULJ; OF THE OPTIC NERVE. 483 



work — both within the stalk itself and outside it — in bringing 

 about the obliteration of the lumen. 



The pressure everywhere outside the stalk is evidently- 

 greater than that within its lumen for, although the first 

 nerve-fibres lie just within the external membrane, the pres- 

 ence of the smallest bundle is. enough to produce a certain 

 amount of bulging of the upper border of the ventral wall 

 into the lumen without in the slightest degree altering the 

 regularity of the outline of the external membrane underneath 

 it (fig. 2). 



It is true that further ingrowth of nerve-fibres produces a 

 considerable change in the outline of the stalk, as shown in 

 fig. 3, but by this time, the lumen has almost been closed, 

 and still further ingrowth of nerve-fibres at the sides, com- 

 pletes its obliteration, and, at the same time, restores the 

 slightly longer axis of the stalk to the horizontal position 

 (figs. 4 and 5). 



IV. Period op Slow Growth, followed by one op Great 

 Activity, consequent on the Formation of the Arach- 

 noid Sheath and the Enclosure op the Subarachnoidal 

 Lymph Space. 



Between the stages shown in figs. 4 — 6, representing 

 tadpoles from 11 mm. to 21 mm. in length, the diameter 

 of the stalk increases only very slightly. This is due to the 

 fact that there is scarcely any karyokinesis going on within 

 the stalk, and the protoplasmic framework, which is now 

 binding the nerve-fibres together, seems unable to accommo- 

 date itself to further expansion. 



Meanwhile, the stalk is being continually more and more 

 stretched between the eye and the brain, so that it is possible 

 to obtain transverse sections of a 21 mm. tadpole that do 

 not contain a single nucleus, only the protoplasmic fibrils 

 proceeding from nuclei that lie in the preceding and suc- 

 ceeding sections. 



